The invention relates to a colour cathode ray tube comprising an in-line electron gun comprising a means for generating three electron beams, a pre-focusing lens portion which contains a first and a second electrode, said first and second electrodes each having three in-line apertures, the apertures in the second electrode being of a substantially circular or square form and a main lens.
Cathode ray tubes of the type mentioned in the opening paragraph are well-known.
In the construction of an electron gun, a number of important parameters must be taken into account, such as the beam displacement (BD). The electron gun has a number of lenses which have a convergent or divergent effect on the electron beams, one of these lenses being the pre-focusing lens, another one being the main lens. A change of the strength of the main lens causes a displacement of the beam on the display screen, this phenomenon is commonly referred to as beam displacement. Problems with the red-blue convergence occur as a result of the beam displacement. These problems adversely affect the picture quality.